Descaling titanium and titanium base alloy articles



United States Patent corporation of Delaware No Drawing. Filed Nov. 21,1958, Ser. No. 775,349 7 Claims. (Cl. 204-141) This invention relates toa method for descaling the surfaces of articles manufactured of titaniumand titanium base alloys, and more particularly to a low temperaturedescaling method which will not raise the hydrogen content of the metalor alloy of which the article is composed.

In the manufacture and fabrication of titanium and titanium base alloyarticles they are often exposed to high temperatures under conditionswhich result in formation of an adherent layer of refractory oxidiccompounds, referred to as scale, on their surfaces. Further processingand ultimate usefulness require removal of such scale.

Several methods have heretofore been employed for descaling the surfacesof titanium and titanium base alloy articles. Baths of molten alkalicompounds have been employed to a limited extent. Such baths apparentlyreact with or loosen certain types of scale so that they may besubsequently removed by mechanical means or acid treatment. Such baths,however, are. expensive to originally provide and to maintain, and mustnecessarily be used above the melting point of the constituent mixturewhich is generally of the order of 480 C. or higher. Such operatingconditions are often dangerous and inconvenient, and in addition suchtemperature may affect the mechanical properties of the metal immersedin such a bath. Moreover scale produced on the surface of certaintitanium alloys does not appear to be amenable to removal by a treatmentemploying molten caustic. In treatment of any titanium article in such abath a danger of hydrogen contamination exists.

Strong acid solutions such as for example, a mixture of nitric andhydrofluoric acid, have also been employed at elevated temperature.These, however, will not successfully loosen or act on heavy andrefractory scale such as is often produced during mill processingandlong exposure of titanium and titanium base alloy articles at hightemperature. The acid strength, treatment temperature, and timenecessary to provide attack on such scale often results in corrosiveaction on the metal itself, resulting in pitting and other detrimentalsurface defects. Acid solutions, employed at elevated temperature andfor the extended period of contact time necessary to act on scale,inevitably raise the hydrogen content of the alloy of which an immersedarticle is composed, to an undesirably high level.

It is therefore a principal object of this invention to provide animproved method for treating scale on the surface of titanium andtitanium base alloy articles. Another object of this invention is toprovide a method for descaling the surface of titanium and titanium basealloy articles that may be operated at low temperature, and

which is more economical, rapid, and convenient than methods heretoforeproposed. Yet another object of this invention is to provide a method ofdescaling titanium and titanium base alloy articles without detrimentalattack on the metal itself and yet at the same time providing efiicientremoval of heavy and refractory scales. A still further object of thisinvention is to provide a method of descaling titanium and titanium basealloy articles without appreciably raising the hydrogen content of themetal of which such articles are composed.

This invention in its broadest aspects contemplates a method fortreating scale on the surface of an article of titanium or titanium basealloy which comprises immersing said article in a sulfuric acidelectrolyte and passing electric current between said article as ananode and a cathode immersed in or contacting said electrolyte until thescale has been loosened or removed, and an anodic film has been formedon the exposed metal surface. The article is then removed from theelectrolyte and the anodic film is removed from its surface, resultingin a. bright, clean and completely descaled titanium or titanium basealloy article. 7

The sulfuric acid electrolyte may be of convenient concentration, andmay be an aqueous solution containing at least 1% H by volume and may beas strong as essentially H 80 It is preferred to employ as anelectrolyte an aqueous solution containing between 40% and 80% sulfuricacid by volume. Concentrations below 40% Will be effective but the timerequired for complete action in the electrolytic descaling step will beunnecessarily lengthened. Concentrations higher than 80% sulfuric acidare not desirable due to the evolution of corrosive sulfur oxide fumes,and will provide little additional reaction speed. The electrolyticdescaling step may be carried out at any temperature, from roomtemperature to the boiling point of the electrolyte. It is preferred tooperate in the range between 80 and C.,

depending on the acid concentration, since this provides adequatereaction speed without excessive loss of electrolyte constituents.

The cell in which the electrolyte is contained may be of convenientconstruction and manufactured of material suitably resistant to thecorrosive effect of the sulfuric acid electrolyte. The cathode employedin the cell may be fabricated of similarly acid resistant metal ormaterial, such as platinum, tantalum, lead, or graphite. If desired, thecell lining itself may be arranged to act as a cathode, and a graphiteliner, which will be acid resistant and a suitable and desirablecathode, may be advantageously employed.

Any suitable conventional and convenient source of direct electriccurrent may be employed to produce the electrolytic action. The positivepole of the current source is connected to the article to be descaledwhich, under such conditions, will be the anode in the cell. Suitableconnection is also made from the negative pole of the power supply tothe cell cathode. Current to the anode and cathode is supplied toprovide a current density over the total immersed surface of the articleto be descaled of between 2 and 100 amperes per square foot, and thevoltage may vary between 3 and 60 volts. At least 2 amperes per squarefoot current density is necessary to provide an anodic protective filmand more than 100 amperes per square foot is not desirable because ofequipment requirements to supply currents of this magnitude and atcurrent densities much above 100 amperes per square foot the acidcontent of the electrolyte has a tendency to decompose. Preferably thecurrent density should be between 5 and 50 amperes per square foot, toprovide efficient current utilization and rapid and effective looseningand removal of the scale.

It is preferred, particularly when employing more concentratedelectrolytes within the defined range, to connect the alloy article andthe cathode of the cell to the electric current supply prior toimmersing the articlein the electrolyte. Also, when removing the articlefrom the electrolyte, to insure that the electric current remainsconshould remain in strong sulfuric acid electrolyte without inspection.

. e3 passage of current betweenthe anode and the cathode some attack bythe electrolytic acid could result in corrosion and pitting of itssurface.

The electrolytic action in the cell operated as described above resultsin loosening or removal of scale, and the formation on the metal surfaceof an anodic film. Such anodic film is itself a form of oxidic coatingbut quite different in character from the original. scale. It is aunique feature of this invention, however, that such anodic film isformed as a result of electrolytic action which loosens or removes thescale. It protects the surface While current passage is maintained toprevent detrimental attack by the electrolyte acid. At' the same time,since the article being descaled is employed as the anode duringelectrolysis, oxygen will be generated and released by electrolyticaction and decomposition of the electrolyte from its surface. Hydrogenwill be correspondingly released at the cathode. Since the article isanodic, no hydrogen will be formed at or near its surface and nohydrogen will therefore be absorbed by the metal of the article as aresult of the descaling action.

Electrolysis in the cell is continued for at least one minute and untilthe scale is eifectively loosened or removed and the anodic film asdescribed is formed on the surface of the immersed article. Since theoriginal scale and the anodic film present a completely differentappearance, completion of descaling may bedetermined by visual In anyevent the time of treatment in the cell will vary between about oneminute and about one hour, when the preferred operating condition ofbetween 5 and 50 amperes current density is employed. Current densitieslower than the preferred range stated will require correspondinglylonger time. Excess time in the cell will do no harm but will notprovide improved results. Light scales maybe removed in a short time,that is, up to 3 to 5 minutes, medium scales may take 5 to minutes andthe heaviest and most refractory scales may take as long as aboutminutes. After the scale has been eifectively loosened or removed, thearticle is removed from the electrolyte. The electric current supply isdisconnected andresidual electrolyte adhering to its surface preferablyremoved, conveniently by a water rinse. If any loosened scale remains onthe article surface that has not been dislodged during the electrolyticprocess, it may be separated by wiping, brushing, or other simplemechanical action.

Removal of the anodic film and loosened scale from the article surfacemay be readily accomplished by mechanical abrasion such as sanding, wirebrushing or grinding, or by chemical means such as immersion in picklingacid foran appropriate time. It is preferred to remove the anodic filmby immersion in a pickling acid for a period of between about one andabout five minutes at a temperature not higher than about 80 C. Thecharacter of the anodic film is quite different from that of the oxidiccompounds of the original scale which are loosened and removed duringthe electrolytic step. The anodic film is readily soluble in acidsolutions containing a fluoride, and a solution of 10% nitric and 2%hydrofluoric acids will be found efiective and advantageous.

Aqueous solutions of other acids such as hydrochloric,

acetic, or sulfuric, in combination with hydrofluoric acid, may also beemployed. Immersion in such acid Will resultin removal of the scale andanodic film and an incidental amount of the metal itself to provide abright, clean descaled surface.

Treatment to remove the anodic film may be intensified as by immersionfor a longer period of time, employment of more concentrated picklingsolution, or at more elevated temperatures. This can result indissolution of a small but appreciable thickness of metal at the surfaceof the article. In so doing the top layer which is contaminated to 'asmall or greater extent by a dissolved oxide content is thereby removed.Such treatment provides surface metal which will be essentially uniformwith the interior metal and thus avoid detrimental effect on mechanicalproperties caused by high oxygen content metal at the surface.

Such purposeful and useful removal of gage should be distinguished fromuncontrolled and detrimental corrosion, pitting, and similar effectscaused by employment of strong acids in heretofore proposed treatmentsfor removal or loosening of scale.

Articles whose surfaces are descaled by the process of this inventionmay be of any size or shape commensurate, of course, with sizelimitations of apparatus, such as tanks and baths. Fabricated articles,castings, and forgings, as well as semi-fabricated products, such asplate, sheet, bars, rods, Wire, tubing, and other shapes, may beadvantageously descaled. The method of this invention is useful fordescaling articles of commercially pure metallic titanium as well as anyof the known commercially useful titanium base alloys such as, forexample, Ti-4% Al3% Mol% V, Ti-6% Al4% V, Ti13% V-11% Cr--3% Al, Ti8 Mnand Ti--2% Fe2% Mo-2% Cr. It will readily and effectively descale thesurfaces of articles of titanium base alloys containing relatively highpercentages of chromium and molybdenum which are dillicult, if notimpossible, to conveniently and readily descale by previously knownmethods.

The following examples illustrate selected embodiments of the practiceof this invention.

Example I An electrolyte consisting of a solution of essentially l( O%sulfuric acid was contained in a small glass tank and maintained atatemperature of between and 100 C. A platinum wire cathode was immersedin the electrolyte and connected to the negative pole of a DC. powersource. A section of sheet of titanium base alloy containing 4%aluminum, 3% molybdenum and 1% vanadium and having a medium scaledsurface result ing from solution heat treatment, was connected to thepositive pole of the D.C. power source and immersed as the anode in theelectrolyte. A current density of about 25 amperes per square foot oftotal surface was maintained by adjusting the power input and thevoltage varied between 10 and 50 volts. After 15 minutes immersion underthese conditions, the surface appearance of the sheet had radicallychanged and the scale had been replaced by an anodic film. The sheetsection was then removed from the electrolyte, the power sourcedisconnected therefrom and residual electrolyte washed olf by means of awater rinse. The sheet section was then immersed in an aqueous solutioncontaining 10% nitric and 2% hydrofluoric acids by volume, for twominutes, at a temperature of about 50 C. After removal from thissolution, residual acid was rinsed ofl with water and the sheet surfaceappeared bright, clean and free from scale or oxide coating. Noappreciable metal thickness was removed and the hydrogen content of thesheet after treatment was determined to be 0.0045% compared to 0.0048%before descaling.

Example 2 The same general process steps as those described for FExample 1 were employed on the same type of scaled sheet except that theelectrolyte was composed of an aqueous solution containing 5% sulfuricacid. Amperage was maintained at about amperes per square foot of sheetsurface and voltage varied from 4 to 54. After 15 minutes in theelectrolytic bath at between 83 and 92 C., the sheet was removed,rinsed, and wiped with a cloth to separate loose scale. The sheet wasthen immersed in a pickling bath similarito that of Example 1, for twominutes and subsequeutly showed asirnilar surface bright, clean and freefrom scale or oxide coating. The hydrogen content of the sheetmetal wasnot increased. 7

Example 3 The same general process steps as those described for Example1 were employed on the same type of scaled sheet except that theelectrolyte was composed of an aqueous solution containing 60% sulfuricacid. Amperage was maintained at about 30 amperes per square foot ofsheet surface and voltage varied from 6 to 23. After 10 minutes in theelectrolytic bath at between 94 and 96 C., the sheet was removed andrinsed to remove adhering electrolyte. The sheet was then immersed in apickling bath similar to that of Example 1, for 2 minutes andsubsequently showed a similar surface, bright, clean and free from scaleor oxide coating. The hydrogen content of the metal was not increased.

Example 4 An electrolyte consisting of an aqueous solution containing60% sulfuric acid was contained in a small glass tank and maintained ata temperature between 90 and 95 C. A platinum wire cathode was immersedin the electrolyte and connected to the negative pole of a DC. powersource. A section of sheet of commercially pure titanium, having lightscale resulting from exposure during annealing, was connected to thepositive pole of the D.C. power source, and immersed as the anode in theelectrolyte. A current density of about 25 amperes per square foot oftotal surface was maintained by adjusting the power input and thevoltage varied from 30 to 52" volts. After 5 minutes immersion underthese conditions the surface appearance of the sheet had radicallychanged and the scale had been replaced by an anodic film. The sheet Wasthen removed from the electrolyte, the power source disconnectedtherefrom and residual electrolyte washed off by means of a water rinse.The sheet section was then immersed in an aqueous solution containingnitric and 2% hydrofluoric acids by volume for 2 minutes at atemperature of about 50 C. After removal from this solution, residualacid was rinsed 01f with water and thesheet surface appeared bright,clean and free from scale or oxide coating. No appreciable metalthickness was removed and the hydrogen content of the sheet metal wasnot increased.

Example 5 An electrolyte consisting of an aqueous solution containing60% sulfuric acid was contained in a small glass tank and maintained ata temperature between 114 and 117 C. A graphite rod cathode was immersedin the electrolyte and connected to the negative pole of a DC. powersource. A section of sheet of titanium base alloy containing 6% aluminumand 4% vanadium, having a medium mill scaled surface, was connected tothe positive pole of the DC. power source and immersed as the anode inthe electrolyte. A current density of about 45 amperes per square footof total surface, was maintained by adjusting the power input, and thevoltage varied between 4 and 6 volts. After minutes immersion underthese conditions the surface appearance of the sheet had radicallychanged and the scale had been replaced by an anodic film. The sheetsection was then removed from the electrolyte, the power sourcedisconnected therefrom, and residual electrolyte washed off by means ofa water rinse. A sander was then employed to remove the anodic filmwhich was readily ground olf by light sanding to provide a sheet surfacewhich appeared bright, clean and free from scale or oxide coating.- Onlya slight amount of metal thick ness was removed during the sandingoperation and the hydrogen content of the sheet metal was not increased.

Example 6 An electrolyte consisting of an aqueous solution containing60% sulfuric acid was contained in a small glass tank and maintained ata temperature between 118 and 128 C. A platinum wire cathode wasimmersed in the 6 electrolyte and connected to the negative pole of aDC. power source. A section of sheet of titanium base alloy containing5% aluminum and 2 /z% tin and having a heavy mill scaled surface, wasconnected to the positive pole of the DJC. power source, and immersed asthe anode in the electrolyte. A cun'ent density of about 45 amperes persquare foot of total surface was maintained by adjusting the power inputand the voltage varied between 4 and 15 volts. After 12 minutesimmersion under these conditions,'the surface appearance of the sheethad radically changed and the scale had been replaced by an anodic film.The sheet section was then removed from the electrolyte, the powersource disconnected therefrom, and residual electrolyte washed off bymeans of a water rinse. The sheet section was then immersed in anaqueous solution containing 10% acetic and 5% hydrofluoric acids byvolume, for 3 minutes at a temperature of 50 C. This treatment removedthe anodic film and also dissolved about 2 mils gage from each side ofthe sheet. After removal from this solution, residual acid was rinsedoff with water and the sheet surface appeared bright, clean and freefrom scale or oxide coating. A strip sample taken from the so-treatedsheet, showed bend properties indicating ductile metal at the surface.The hydrogen content of sheet metal was not appreciably increased.

Example 7 An electrolyte consisting of an aqueous solution containing60% sulfuric acid was contained in a small glass tank and maintained ata temperature between and C. A platinum wire cathode was immersed in theelectrolyte and connected to the negative pole of a DC. power source. Asection of sheet of titanium base alloy containing 13% vanadium, 11%chromium and 3% aluminum and having a heavy mill scaled surface wasconnected to the positive pole of the DC. power source and immersed asthe anode in the electrolyte. A current density of about 50 amperes persquare foot of total surface, was maintained by adjusting the powerinput and the voltage varied between 10 and 38 volts. After 15 minutesimmersion under these conditions the surface appearance of the sheet hadradically changed and the scale had been replaced by an anodic film. Thesheet section was then removed from the electrolyte, the power sourcedisconnected therefrom, and residual electrolyte washed off by means ofa water rinse. The sheet section was then immersed in an aqueoussolution containing 10% nitric and 2% hydrofluoric acids by volume for 2minutes at a temperature of about 50 C. After removal from this solutionresidual acid was rinsed off with water and the sheet surface appearedbright, clean and free from scale or oxide coating. No appreciable metalthickness was removed and the hydrogen content of the sheet was notincreased.

The mechanism by which the scale is loosened or removed in theelectrolytic treatment is not clearly understood. It is theorized thatpossibly the-scale is to some extent porous or reactive with theelectrolyte allowing the electrolyte acid to penetrate the film and torelease oxygen on contact with sub-scale metal. The oxygen maymechanically loosen and separate the scale or react with it chemicallyto destroy its cohesive and adhesive characteristics. At the same timean anodic film, which is, as is well known, a type of oxide coating, isformed on the metal surface protecting it against any detrimental attackby the electrolyte acid. Thus the method of this invention provides aprocess for treating scale on the sur face of a titanium or titaniumbase alloy article in which tenacious and refractory scale oxide isconverted to an easily removable oxide by immersing in an acid bathwhile maintaining anodic protection of the metal surface. The process isoperated at low temperature, compared to those of molten alkali bathspreviously employed to attack stubborn scale, and the sulfuric acid andpower employed are relatively inexpensive. The electrolyte is moreconvenient and safer than molten alkalis. The process of this inventionwill remove the heaviest and most're sistant scales on titanium metal orany type of titanium base alloy without detrimental attack on the metalitself, and the characteristic efficiency of this action without raisingthe hydrogen content of the metal is extremely advantageous.

I claim:

1. A method for descaling the surface of an article of metal selectedfrom the group consisting of titanium and titanium base alloys whichcomprises; immersing and maintaining said article as an anode in asulfuric acid electrolyte of concentration more than 1% H 80 by Volume,meanwhile passing electric current at between 3 and 60 volts betweensaid article and a .cathode to provide a current density of betweenabout 2 and about 1% amperes per square foot of said surface for aperiod of at least one minute, at a temperature of between roomtemperature and the boiling point of said electrolyte, thereby to loosenand remove scale from said surface of said article and to form on saidsurface an anodic film, removing said article from said electrolyte andremoving said anodic film from said surface thereof.

2. A method for descaling the surface of an article of metal selectedfrom the group consisting of titanium and titanium base alloys whichcomprises; immersing and maintaining said article as an anode in asulphuric acid electrolyte of concentration between 40% and 80% H 50 byvolume, meanwhile passing electric current at between 3 and 60 voltsbetween said article and a cathode to provide a current density ofbetween about 2 and about 100 ampcres per square foot of said surfacefor a period of at least one minute, at a temperature of between roomtemperature and the boiling point of said electrolyte, thereby to loosenand remove scale from said surface of said article and to form on saidsurface an anodic film, removing said article from said electrolyte andremoving said anodic film from said surface thereof.

3. A method for descaling the surface of an article of metal selectedfrom the group consisting of titanium and titanium base alloys whichcomprises; immersing and maintaining said article as an anode in asulphuric acid electrolyte of concentration more than 1% H 80 by volume,meanwhile passing electric current at between 3 and 60 volts betweensaid article and a cathode to provide a current density of between about5 and about 50 amperes per square foot of said surface for a period ofat least one minute, at a temperature of between room temperature andthe boiling point of said electrolyte, thereby to loosen and removescale from said surface of said article and to form on said surface ananodic film, removing said article fiom said electrolyte and removingsaid anodic film from said surface thereof.

4. A method for descaling the surface of an article of metal selectedfrom the group consisting of titanium and titanium base alloys whichcomprises; immersing and maintaining said article as an anode in asulphuric acid electrolyte of concentration more than 1% H 80 by volume,meanwhile passing electric current at between 3 and 60volts between saidarticle and a cathode to pro vide a current density of between about 2and about 100 amperes per square foot of said surface for a period ofbetween about one minute and about one hour, at a temperature of betweenroom temperature and the boiling point of said electrolyte, thereby toloosen and remove scale from said surface of said article and to formtitanium base alloys which comprises; immersing and maintaining saidarticle as an anode in a sulphuric acid electrolyte of concentrationmore than 1% H by volume, meanwhile passing electric current at between3 and 60 volts between. said article and a cathode to provide a currentdensity of between about 2 and about 1.00

arnperes per square foot of said surface for a period of.

at least one minute, at a temperature of between room temperature andthe boiling point of said electrolyte, thereby to loosen and removescale from said surface of said article and to form on said surface ananodic film, removing said article from said electrolyte and removingsaid anodic film from said surface thereof by immersion in a picklingacid. I

6. A method for descaling the surface of an article of metal selectedfrom the group consisting of titanium and titanium base alloys whichcomprises; immersing and maintaining said article as an anode in asulphuric acid electrolyte of concentration more than 1% H 80 by volume,meanwhile passing electric current at between 3 and 60 volts betweensaid article and a cathode to provide a current density of between about2 and about amperes per square foot of said surface for a period of atleast one minute, at a temperature of between room temperature and theboiling point of said electrolyte, thereby to loosen and remove scalefrom said surface of. said article and to form on said surface an anodicfilm, removing said article from said electrolyte and removing saidanodic film from said surface thereof by immersion in an aqueoussolution containing about 10% nitric and about 2% hydrofluoric acids byvolume for a period of from one to five minutes at a temperature up toabout 80 C.

7. A method for descaling the surface of an article of metal selectedfrom the group consisting of titanium and titanium base alloys whichcomprises; immersing and maintaining said article as an anode in asulphuric acid electrolyte of concentration more than 1% H 80 by volume,meanwhile passing electric current at between 3 and 60 volts betweensaid article and a cathode to provide a current density of between about2 and about 100 amperes per square foot of said surface for a period ofat least one minute, at a temperature of between room temperature andthe boiling point of said electrolyte, thereby to loosen and removescale from said surface of said article and to form on said surface ananodic film, removing said'article from said electrolyte and removingsaid anodic film from said surface thereof by mechanical abrasion.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Metallurg i'a, vol. 49, No. 295, May 1954, page 256. 1Handbook on Titanium Metal, 7th ed., Titanium Metals Corporation ofAmerica, New York, pages 88-89, 92, and 93.

Dailey Feb. 5, 1957

1. A METHOD FOR DESCALING THE SURFACE OF AN ARTICLE OF METAL SELECTEDFROM THE GROUP CONSISTING OF TITANIUM AND TITANIUM BASE ALLOYS WHICHCOMPRISES: IMMERSING AND MAINTAINING SAID ARTICLE AS AN ANODE IN ASULFURIC ACID ELECTROLYTE OF CONCENTRATION MORE THAN 1% H2SO4 BY VOLUME,MEANWHILE PASSING ELECTRIC CURRENT AT BETWEEN 3 AND 60 VOLTS BETWEENSAID ARTICLE AND A CATHODE TO PROVIDE A CURRENT DENSITY OF BETWEEN ABOUT2 AND ABOUT 100 AMPERES PER SQUARE FOOT OF SAID SURFACE FOR A PERIOD OFAT LEAST ONE MINUTE, AND A TEMPERATURE OF BETWEEN ROOM TEMPERATURE ANDTHE BOILING POINT OF SAID SURFACE OF THEREBY TO LOOSEN AND REMOVE SCALEFROM SAID SURFACE OF SAID ARTICLE AND TO FORM ON SAID SURFACE AN ANODICFILM, REMOVING SAID ARTICLE FROM SAID ELECTROLYTE AND REMOVING SAIDANODE FILM FROM SAID SURFACE THEREOF.